The present invention relates to acousto-optic systems for performing basic operations of vector-matrix multiplication with digital (16-bit) accuracy. More specifically the present invention relates to an acousto-optic system comprised of two multi-transducer Bragg cells that can perform at least 10.sup.8 operation/sec of basic vector-matrix multiplication with digital (16-bit) accuracy and which is simple and compact.
Optical processors have the advantages of (a) inherent parallelism of data propagation; (b) ease of performing analog multiplication and additions, and (c) the use of a third spatial dimension to achieve global interconnections between different elements of an array. One generally useful class of operations which inherently require these three properties is that of matrix operations--e.g. vector-matrix and matrix-matrix multiplication. A review of different architectures for optical matrix processors can be found in the literature (R. A. Athale, "Optical Matrix Algebraic Processors: A Review," Proceedings of the 10th International Optical Computing Conference, p. 24, IEEE Catalog No. 83 CH 1880-4). Optical computing is however largely analog. Recently several papers presented ways to improve the accuracy by digital encoding of data--especially as configured for a matrix processor (R. A. Athale, W. C. Collins, P. D. Stilwell, Appl. Opt, Vol. 22, p. 368, 1983; P. S. Guilfoyle, Optical Eng., Vol. 23, p. 20, 1984). In addition to these architecture developments, recently the technology for manufacturing large bandwidth (.about.100 MHz), high performance, multichannel acousto-optic devices for rapid input of data into optical processors has matured considerably ("Acoustooptic Signal Processing Theory and Applications," N. J. Berg and J. N. Lee, eds. Marcel-Dekker Pub., New York, 1983). The wide variety of architectures that are possible with optical systems allow for the desired input/output interfacing to be achieved.
The basic operation chosen for the present optical system architecture is the multiplication of two vectors to produce a scalar (inner or dot product). The vector-matrix multiplication between an N element column vector and an MxN matrix can be accomplished in M steps as successive inner products between the input vector and columns of the matrix ##EQU1## A matrix-matrix multiplication can be accomplished in N.sup.2 steps by a straight forward extension. For higher accuracy the algorithm used will be digital multiplication by analog convolution first proposed by Whitehouse and Speiser (H. J. Whitehouse and J. M. Speiser in "Aspects of Signal Processing with Emphasis on Under Water Acoustics," Vol. 2, Reidel Pub., Boston, 1976, p. 669-702) and first demonstrated optically by D. Psaltis et al. (D. Psaltis, et al., proc. of SPIE, Vol. 232, p. 160-167, 1980). This principle was combined with the optical systolic architecture for vector-matrix multiplier (H. J. Caulfield, W. T. Rhode, M. J. Foster and S. Horvitz, "Optical Implementation of Systolic Array Processing,"Optics Communications, Vol. 40, No. 2, 15 December 1981, pp. 86-90. as well as optical engagement architecture in a paper by P. S. Guilfoyle referred to earlier. The proposed system in this paper has several drawbacks, however. It requires acousto-optical devices with two different materials. The acoustic velocity ratios are not easily adjustable, and therefore the accuracy of the system which is largely determined by those ratio's is difficult to adjust. The optical components required to image two Bragg cells, the commonly used acousto-optic device in the above-identified systems, on each other require a rather large anamorphism (.about.16) thus making it complicated. The output interface requires a linear array of N high speed detectors and each detector requires an analog/digital converter and a digital shift-and-add circuit to decode the output, making it very expensive. The output vector is calculated after (3N-1) steps of the processor.